A method for checking an exhaust gas catalyzer of an internal combustion engine is known from DE 43 328 38 C2. The exhaust gas catalyzer is arranged in an exhaust gas duct of the internal combustion engine. The internal combustion engine features a Lambda control device which comprises a Lambda probe arranged upstream of the exhaust gas catalyzer and regulates an air-/fuel ratio to a predetermined setpoint value. To check the conversion capability of the exhaust gas catalyzer, in stationary operation with the internal combustion engine at operating temperature, an oxygen charge of the exhaust gas catalyzer is increased by a control intervention of the Lambda control device during a diagnosis period up to a predetermined value. During the diagnosis period the NOX concentration in the exhaust gas duct upstream of the 3-way exhaust gas catalyzer is detected using an NOX sensor. A stationary diagnosis value is determined from values of the NOX concentration during at least one Lambda controller oscillation. The diagnostic value is compared to a predetermined threshold value and, if the threshold value is exceeded, it is concluded that the exhaust gas catalyzer has aged.